Small tweaks and cleanup, roxygenize and import instead of depends

As discussed with @RaymondMcT in e-mail, I have tweaked the package a bit. Please review and feel free to merge or ignore my commits here. I tried to make short and separate commits so that the review could be any easy process, but in short, I've done the following stuff:

 * cleaning-up unnecessary elements (e.g. the `.DS_store` files might go to `.gitignore`, not sure about your added but not used `Rdata` and some lines of code like the `library` calls and `new.env`)
 * roxygenized the docs so that you would not need to hand-edit the `Rd` files. See the references in the commit message.
 * I did not want to touch your coding sytle, but after much refactoring I decided to make it closer to what Hadley suggested in his nice wiki (pls see my commit msg for references)
 * and removed the packages from the `Depends` field of your `DESCRIPTION` file and rather `Importing` package function now to keep the clutter of function names at a minimal level in the `.GlobalEnv`.

As far I as tested, the package works fine after my modifications, but please be sure to review and test it. And sorry for much modifications, I hope you do not mind. Maybe a version bump should have also been attached, but I did not want to tweak your `DESCRIPTION` file any further.